A light emitting diode (LED) is a kind of a semiconductor device capable of converting electric energy into light. The light emitting diodes have many advantages over conventional light sources, such as a light bulb or a fluorescent light, including lower energy consumption, longer lifetime, faster response speed, improved safety, and better eco-friendly characteristic. A lot of studies have been conducted recently in order to replace the conventional light sources by the light emitting diodes. The light emitting diodes are increasingly used as a light source of various indoor/outdoor lighting devices, for example, lamps, liquid crystal devices, electronic display boards, or streetlamps.